Wireless sensor networks (WSNs) include a plurality of spatially distributed intermediate nodes and leaf nodes that transmit data to a central node or a root node by way of the intermediate nodes. Some of the leaf nodes include sensors that monitor physical or environmental conditions, such as temperature, sound, and pressure and transmit data indicative of these parameters through the network to the central node. For example, a WSN can be installed in a forest to detect when a fire has started. Some of the leaf nodes and/or intermediate nodes can be equipped with sensors that measure temperature, humidity, and gases that are produced by fires in trees or vegetation. The measured data is transmitted by way of intermediate nodes to a central node, which transmits the data to a data processor, so that early fire detection is accomplished.
A WSN typically has a plurality of branches wherein a leaf node is the furthest node on a branch from the central node. Before a leaf node can transmit data, it must join the WSN. When a leaf node attempts to join a branch of a WSN, the leaf node monitors beacons transmitted from intermediate nodes to acquire time slot information, so it can transmit an “association request” during a shared time slot. Two types of time slots are used in WSN transmissions, dedicated time slots and shared time slots. Dedicated time slots have channels that are for transmitting data between specific node pairs. A dedicated time slot has a dedicated channel that has been assigned for communications between two nodes during the dedicated time slot. Shared time slots have contention based channels that can be used by any node. During a shared time slot, for the most part, any two nodes may use channels during the shared time slot. A dedicated time slot may be dedicated between two nodes in that the two nodes communicate on a specific channel during that time slot. All association requests are transmitted solely during shared time slots.
Once time slots information is obtained by the leaf node, the leaf node transmits the association request to the intermediate node from which it received the information using a shared time slot. The intermediate node then forwards the association request to a node closer to the root node also using shared time slots. The process continues until the association request reaches the root node. When the root node receives the association request, it processes the request, assigns a dedicated time slot between the leaf node requesting association and its corresponding intermediate node. The root node then includes this information in an association response that it sends to an intermediate node, which forwards the information to other intermediate nodes until the information reaches the leaf node. All the association response data is sent to the leaf node during shared time slots.
Sending association requests during shared time slots is not efficient because the association requests are susceptible to data collisions, which can result in the association requests needing to be retransmitted. Transmitting association requests consumes a great deal of power, so retransmitting association requests reduces the power efficiency of the WSN. As described above, the association requests are transmitted during the shared slots, so some of the intermediate nodes may not be listening during the shared time slots to save power, which can result in further delay of the node association. Moreover, additional association requests are required to be transmitted because the aforementioned nodes are not listening during the shared time slots. The transmission of additional association requests reduces efficiency.